This invention relates to ultrasonic fluid flowmeters and, more particularly, to a method and apparatus for measuring and controlling fluid flow in small diameter conduits utilizing ultrasonic techniques.
Many flowmeters have been developed in recent years for measuring both flow rate and sound speed within a fluid. A particular advance in this area was made with the advent of ultrasonic measuring techniques such as the so-called sing-around velocimeter developed at the Bureau of Standards in the 1950's. This velocimeter is described in an article by Greenspan et al. Review of Scientific Instruments 28, No. 11, November 1957. More recently, ultrasonic velocimeters and flowmeters utilizing phase-lock techniques have been developed for making sound speed and fluid flow measurements. Exemplary of such devices and methods are those described in U.S. Pat. No. 3,780,577 issued Dec. 25, 1973 to Alvin E. Brown. Many of these ultrasonic devices have been used in flow measurements of fluids including liquids as well as gases in pipelines and conduits. A particular problem, however, has arisen if the flow is to be measured is in a conduit having a small inside diameter or bore, such as those used in many instrumentation systems, e.g., liquid chromatographs. In such systems, the flowmeter transducers, whether operating in an axial mode or a radial mode relative to the conduit, cannot without extreme difficulty, if not impossibility, be placed in a meaningful position with respect to the conduits to permit accurate measurements. This is true not only because of the small diameter, but also because of the low flow rates typically encountered.
The so-called Doppler effect flowmeters offer a partial solution to this problem. The transducers of such flowmeters can be placed external to the conduits and, utilizing backscatter techniques, measure the flow rate within the conduit. Doppler flowmeters of this type are described, for example, in U.S. Pat. No. 3,896,788 issued July 29, 1975 to Yutaka Sato, and U.S. Pat. No. 3,575,050 issued Apr. 13, 1971 to Lawrence C. Lynnworth. Theoretically, such Doppler effect flowmeters can be placed external to the conduit, even one having a small diameter, and the flow therein measured. Unfortunately, however, flow measurements made using the Doppler effect are sound speed dependent. Hence in systems wherein the makeup of the fluid being measured is subject to change, such as particularly occurs in instrumentation type systems, the fluid measurements made are in error because of these variations in sound speed. Sound speed of a fluid, of course, varies as a function of fluid temperature, pressure and similar physical as well as chemical variables. The industry has therefore been left with the dilemma that it is difficult to make accurate flow measurements in small bore conduits utilizing ultrasonic techniques.
Accordingly, it is an object of this invention to provide a method of measuring fluid flow in small bore conduits.
Another object of this invention is to provide an improved apparatus for measuring fluid flow in small bore conduits.
A further object of this invention is to provide an improved method and apparatus for controlling the flow rate in small diameter conduits.